1. Technical Field
The present invention relates to a microwave heating apparatus for heating foodstuff, specifically a contraction for heating foodstuffs efficiently and evenly.
2. Description of Related Art
There have been a number of proposals for apparatuses to heat an object in a heating cavity evenly, or efficiently. The following is a description of Japanese Laid-open Patent No. 8-17569, which discloses an apparatus having a construction that is similar to that of the present invention. As shown in FIG. 9, FIG. 10 and FIG. 11, a prior art microwave heating apparatus comprises a heating cavity 22 for heating/cooking a heating object 21, a magnetron 23 for irradiating microwaves into the heating cavity 22, a wave guide 24 for coupling the heating cavity 22 and the magnetron 23, a dielectric axle 25 being inserted in the wave guide 24, a metal reflector 26 held by the dielectric axle 25 for adjusting the impedance comprised of a reflecting surface 26a and a metal stub 26b provided at the circumference of the reflecting surface, and a control means 27 for controlling the positioning of metal stub 26b by revolution of the metal reflector 26. A means for discerning, by way of detecting the temperature of heating object 21, a transition point at which physical properties of a heating object 21 change during microwave heating procedure is provided. Also, the impedance matching of microwaves within heating cavity 22 is varied by revolving the metal reflector 26 before and after the transition point in order to create a heating condition that is optimal for each of various stages of the physical properties of the heating object 21. The apparatus is further provided with a construction that allows it to thaw a frozen heating object 21 by even heating in a state under which the microwaves hardly hit directly on the heating object. Such a state is created by revolving the metal reflector 26, while supplying electric power without interruption to a microwave generating means in order to maintain the operation at a weak output level.
It is true that in the above described prior art microwave heating apparatus, where the impedance is adjusted for a heating object 21, the heating efficiency is improved as compared with a microwave heating apparatus, having a heating cavity 22 of the same shape and the same power supply system with no means for adjusting the impedance. However, with the impedance adjustment means the greatest output of magnetron 23 is being delivered to heating cavity 22, and not all of the output is absorbed into heating object 21. Further, a certain impedance adjusted relative to a heating object 21 is not always one which results in even heating. Therefore, in some instances efficient but uneven heating may occur depending on the category of heating object 21.